Re: 1969 merc 500 50hp idle issues
Mike, running the idle mixture screws out to 4 turns should have killed the motor.
Is there any change in idle speed when you adjust the screws? If not, sounds like the carbs' idle circuits are plugged up and need to be cleaned out.
You'll want to remove the carbs, pull open up the 'float chamber' and clean/adjust float level; remove the main jet brass plug at front bottom of carb and flush thru with carb cleaner then compressed air; remove the idle mixture needles and spray thoroughly with carb cleaner then blow out with air.
Check to see if you can feel air blowing thru the tiny openings in the throat of the carbs, on the engine side of the butterflies. If you can't feel air (or get carb spray) thru these holes when spraying into the idle passage (where the mixture needles go), something's plugged up and needs further cleaning.
Pick up a couple of "carb packing kits", they're inexpensive and will have the carb gasket, float chamber gasket, and other small consumable gaskets you'll need to put the carb back together. Rebuild kits not necessary unless the carb is a total mess inside.
Note there's one thing you can try before tearing the carbs apart, pull both idle mixture needles and give a stout spray into the idle passages with carb cleaner. Then a good puff of compressed air. Sometimes this will clear a blockage. If it doesn't work you've lost nothing but a little time. Note the compressed air is gonna push some gas out the carb, so throw a paper towel or rag over the float chamber and carb throat to catch any spray.
HTH & if that doesn't do the trick let us know......ed
